In recent years the trend in the coatings industry has been toward resin compositions which can be applied using water as the sole or major solvent with at most minor amounts of water-miscible organic liquids. In addition to the cost reduction achieved by employing water in place of more expensive organic solvents such as aromatic and aliphatic hydrocarbons and ketones, aqueous coating compositions are desirable because they generate minimal quantities of pollutants when the solvents are volatilized during baking, a conventional operation in the preparation of cured coatings.
U.S. Pat. No. 3,336,253 discloses potentially water-soluble reaction products of mono- or dialkanolamines with a variety of water-insoluble polymers, particularly epoxide polymers, containing end groups which are reactive with amines. The reaction products are converted to water-soluble materials following neutralization with an acid. The preferred products contain one unreacted epoxide radical per molecule, and are applied as coatings to various substrates, including glass fibers. The coatings are subsequently crosslinked by self-polymerization of the unreacted epoxide radicals. The presence of these epoxide radicals is detrimental to storage stability as they can react further, and may yield materials that are too viscous for use as coating materials. The aforementioned U.S. Pat. No. 3,336,253 teaches that the storage stability of epoxidealkanolamine products can be increased by reacting all of the epoxide groups using a variety of compounds, including additional dialkanolamine. This procedure is not desirable, since it would theoretically leave no sites for subsequent crosslinking considered necessary to obtain durable, solvent-resistant coatings. The patent discloses that coatings of these uncrosslinked polymers were considerably less adherent to metal substrates than cured coatings.
An objective of this invention is to provide a method for converting stable aqueous compositions to flexible, adherent and solvent-resistant coatings for metal substrates, including containers.